The present invention relates to a method and a device in the drying section of a paper machine or the like, such as a board machine or a finishing machine.
Then the invention relates particularly to a method and a device in which    the web is conveyed, supported by a supporting fabric such as a wire or a felt, over a cylinder, such as a drying cylinder, a roll, or the like, between the cylinder and the supporting fabric,    supported by the supporting fabric the web is guided from the opening nip between the cylinder and the supporting fabric toward a roll, such a suction roll, a turn roll, a wire guide roll, another cylinder, or the like, and in which    the run of the web from the opening nip toward said roll is supported by a negative pressure created on that side of the wire which is opposite the web.
The invention is particularly intended to be applied in the drying sections of paper, paperboard or finishing machines or the like. The intention is then to be able to apply the invention in drying sections provided with a single wire or a twin wire run, where a wire pocket is formed between two drying cylinders and a roll below them redirecting the wire travel. An intention is also to be able to apply the invention in drying sections provided with a so called inverted run, i.e. in such drying sections where the roll redirecting the wire travel is arranged above the drying cylinders, or in solutions where drying cylinders are arranged above each other on two or more levels. Further the intention is to be able to apply the invention in drying sections provided with combinations of the above mentioned drying sections. The intention is further to be able to apply the invention in suitable respects in other parts of the above mentioned machines.
Previously it has been noted that there is a great need for a negative pressure in the wire pocket, particularly at the opening gap between the drying cylinder and the wire, in order to be able to ensure that the wire comes off from the surface of the drying cylinder. However, increasement of the negative pressure in the whole pocket to the required negative pressure level will cause certain disadvantages. Large amounts of energy must be used when the whole pocket space must be brought to the same high negative pressure level. Large air leaks may make it impossible to reach a sufficiently high negative pressure and to maintain it. So far it has generally been possible to increase the negative pressure sufficiently with the aid of blow boxes.
Further, increasing the negative pressure of the whole pocket to a high negative pressure level may cause other disadvantages. On long wire runs with the length of the pocket height a high negative pressure may bend the wire and the web. Thus the wire can come to touch the surfaces of the blow box or other inflexible surfaces, which causes wire damages and impairs the runability. The central part and edge parts of the web may bend in different ways, which causes stretching in the web. This impairs the runability. Further it has been noted that a high negative pressure at the opening nip may shift the wire disengaging point higher on the drying cylinder.
An aim has been to secure the travel of the web in the opening gap between the drying cylinder and the wire by increasing the draw in the paper web. Draw means that a velocity difference is used to create tension in the web. However, it is not always possible to increase the draw, because a too high draw will decrease the tensile strength of the paper, impair the paper quality, and often impair the runability, i.e. create more web breaks.
Previously it has also been proposed to arrange a special suction box at the opening nip between the cylinder and the wire to create a higher negative pressure. The American patent publication U.S. Pat. No. 5,341,579 proposes to arrange a particular small suction box at the opening nip, with which a certain negative pressure is maintained at this point. The negative pressure at this suction box 20 and the suction roll 12 is created by the same negative pressure blower 32. Thus they can not be controlled separately.
U.S. Pat. No. 5,782,009 presents a suction box mounted in the pocket between two drying cylinders, whereby the suction box is divided into two sections. The suction box section 1 having a higher negative pressure is arranged in the region of the disengaging point between the drying cylinder and the wire. The region is separated from the environment with the aid of mechanical seals. In the cross direction of the web the section 1 with the higher negative pressure can be divided into several parts, where different negative pressures can be created in order to secure the travel of the edges of the web.
U.S. Pat. No. 4,359,827 presents a multi-section suction box arranged in the pocket formed between two drying cylinders. One section of the suction box is arranged in front of the wire at the first drying cylinder regarding the travelling direction of the wire, before the disengaging point between the drying cylinder and the wire. A higher negative pressure is arranged in this section of the suction box than in the other sections of the suction box which border on the wire.